Started some unit tests.

This is still very incomplete. Also, one of the tests is only
half-written, and is thus failing.
Wrote a test concerning duplicate vlans.

Currently the test is failing.

In addition, I added a docstring to the other test.
duplicate vlan test is now passing.
test_exist
Added a test that checks it throws a NotExistError when adding a node to an non-existing group

The test is passing
Still need more work to handle the exception
Merge pull request #20 from CCI-MOC/unit-tests

I'm pulling in the unit testing stuff, we should start using this on other feature branches.